The derivation of the placename is given in the Life of Saint Máedóc of Ferns compiled in 1536 from earlier sources, where it is spelled as Coill na gCros.
The text states, Aed Dub (the Black) son of Fergna, son of Fergus, the king of the Ui Briuin, heard of the manifold miracles done by Maedoc, and the honour and great reverence which he had received at Rome, and the two noble and illustrious gifts which he had received there, the variegated (crozier) and the staff.
He came zealously to him, and fasted humbly to him (asking him) to obtain from God for him a change of form and feature, for up to that time he was hideous.
He was afterwards baptized at Ath Airm (Weapon Ford) in Coill na gCros (Wood of the Crosses); and it is from the marking of the king with the Cross, and his consignation, that the place is called Coill na gCros, and Ath Airm from the weapons and dress of the king which were taken from him while he was being baptized and blessed (and were given) in perpetuity to God and to Maedoc.
Aed the Fair, from that day forth There came, by virtue of his baptism, The two Breifnes under the saint's authority, Not by way of refusal of either part.
[6] Thirdly the Uí Briúin Bréifne had no presence in County Cavan until about two hundred years after the time Aedh Dubh lived.
In the Dúchas folklore collection a similar tale is told to explain the name of Kilnagross townland in County Leitrim.
On 1 May 1611 James Craig leased, inter alia, "1 poll of Killingrosse to Eugene mac Cahell McKernan".
James Craige is their deputy for five years, who has brought 4 artificers of divers sorts with their wives and families and 2 other servants.
[11] An Inquisition held at Ballyconnell on 2 November 1629 stated that the poll of Kyllnagrosse contained nine sub-divisions named "Towneymuckkellagh, Tawnegrassey, Mullaghdrissogagh, Cortubber, Tawnenichall, Tawnegarrawe, Mullaghdeavagh, Lismeaghenbonilagh and Moyghreigogartie".